So, how serious is the legal battle between Apple and the various Android phone makers, really? Surely, it's just logical business sense that's behind it, right? Calculated, well-planned precision strikes designed to hurt Android where simply making better, more innovative products isn't enough? Well, no, not really. We already knew Steve Jobs took this personal - now we know just how personal.

Mac OS X uses a different kernel called Mach, which was developed from scratch by Avie Tevanian, a NeXT engineer.

Not true. NeXTStep kernel was based on Mach 2.5 microkernel, which was NOT made by Apple or NeXT engineers but written by Carnegie Mellon University.
Thanks to give credits where it's due.

Also, over the years, most of the UNIX based system tools which run under the hood in Mac OS X (and are part of Darwin) have been replaced with Apple-developed alternatives. Launchd is one small example, to replace the cron/init stuff from UNIX. A bigger example is the compiler, where GCC has been replaced by Clang/LLVM.

Clang/LLVM was not started by Apple but at University of Illinois and release under open-source licence, which latter allow Apple to 1) hire one of the student behind LLVM and 2) reuse and improve the code without breaking the licence. Otherwise, they will have to start again from scratch, which they didn't have to, thanks for the open source output of others, like for the Mach kernel.

Again, thanks to give credits where it's due.

The only thing which is more or less the same consistently between UNIX, Linux and Mac OS X are the BSD userland tools and X11 window system, which make you able to run UNIX and X11 applications on these systems. Both are an optional install for Mac OS X and not really required to boot the system.

Try to remove the Mach kernel and see how fine it boot, just for fun.
The NIH syndrom (Not Invented Here) is not an excuse to
NOT give credits where its due.
Worst, when it make someone hide or voluntary forget the actual history track is a bit sad but, to be straigth, also a kind of revisionism.

Facts are facts.
Apple innovates? True.
Apple wrote all her stuffs from scratch? False.
Apple own some credits to others works? True.